Excimer formation in the mixed dimers of naphthalene and 1-methoxynaphthalene in a supersonic jet

Abstract

Electronic spectra for the S₁←S₀ transition of jet-cooled mixed clusters of naphthalene and 1-methoxynaphthalene have been measured by laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. Under jet-cooled conditions, naphthalene forms two isomeric 1:1 mixed dimers with 1-methoxynaphthalene, which exhibit significant differences in excimer formation dynamics from the locally excited states. One of the isomeric dimers emits excimer fluorescence when excited to the electronic origin of the lowest excited state but the other shows a significant barrier to formation of the excimer. The energy gap between the S₁ states of naphthalene and 1-methoxynaphthalene is ∽350 cm⁻¹. The observation of excimer emission, particularly when excited to the S₁ origin of one of the isomeric dimers, supports our earlier proposal that for naphthalene–methoxynaphthalene mixed dimers energy exchange or exciton resonance interactions are not significant for the stabilization of the mixed excimers. The charge-transfer (CT) interaction and overlap between π-molecular orbitals of two molecules at a suitable geometry are considered to be the primary factors for the stability of the excimers. At higher naphthalene vapour pressure we have observed a mixed trimer of 2:1 composition of naphthalene and 1-methoxynaphthalene. The trimer does not emit excimer fluorescence when excited with additional vibrational energy in S₁ up to 650 cm⁻¹. The observation indicates that the geometry of the trimer is very different from the stack configuration essential for formation of the excimer.